Experimental evidence for maternal offloading of mercury pollution via reproduction

Of the many pollutants released into the environment by human activities, mercury is among the most toxic. Interestingly, studies typically find reproducing females to exhibit lower mercury concentrations than males, suggesting that maternal elimination of this toxic compound occurs via transfer to developing embryos. Yet, clear evidence for the causality of such an effect is lacking, because of the absence of studies that manipulated reproductive activity. We experimentally manipulated reproductive status in an avian model organism, the Japanese quail (Coturnix japonica), and exposed reproductively active and inactive males and females to a single sub-lethal dose of mercury. We found that reproductively active (egg laying) females had considerably lower levels of blood mercury a week after their experimental exposure to this compound, compared to inactive females and males, and that this difference was maintained for at least two more weeks. Moreover, we found maternal mercury to end up in their eggs and chicks, at levels that gradually decreased the later these eggs and offspring were produced after maternal mercury exposure. As such, we provide experimental support for maternal offloading of mercury to eggs as a cause for lower mercury levels in reproductively active females. As this causes offspring to be exposed to mercury at a stage where they are most vulnerable, these findings emphasize the importance of considering intergenerational effects when assessing the ecological impacts of anthropogenic pollution on wildlife.